title: Opposing effects of N-ethylmaleimide on the affinity of carbachol for muscarinic cholinoceptors of guinea-pig atrium.
abstract: 1. Inhibition of the binding of [3H]quinuclidinyl benzilate to homogenates of guinea pig right atrium (M2 receptors) by varying concentrations of carbachol was studied. 2. Pretreatment of membranes with 5 x 10(-5) M N-ethylmaleimide at 2 degrees C shifted the carbachol inhibition curve to the right, indicating decreased affinity of the receptor for carbachol. However pretreatment at 37 degrees C moved the curve to the left. 3. The ability of guanyl-5'-yl imidodiphosphate to reduce agonist affinity was largely eliminated by treatment with N-ethylmaleimide at both temperatures. 4. Conflicting reports in the literature and the present results can be explained by invoking a model in which N-ethylmaleimide has a high affinity for a heat-labile site and a lower affinity for a heat-insensitive site. Reaction with the first site decreases agonist affinity, but at 37 degrees C this site is largely inactivated and reaction with the second site, which leads to increased agonist affinity, predominates.
authors.affiliation: Materials Research Laboratory, DSTO, Victoria, Australia.
authors.forename: R M
authors.initials: RM
authors.lastname: Dawson
authors.forename: M
authors.initials: M
authors.lastname: Poretski
mesh_terms: D000818:Animals; D002217:Carbachol / Q000494:pharmacology*; D005033:Ethylmaleimide / Q000494:pharmacology*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology*; D006168:Guinea Pigs; D006325:Heart Atria / Q000378:metabolism*; D011292:Premedication; D011813:Quinuclidinyl Benzilate / Q000378:metabolism*; D011950:Receptors, Cholinergic / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000378:metabolism; D013696:Temperature
chemical_list: D011950:Receptors, Cholinergic; D011976:Receptors, Muscarinic; D006165:Guanylyl Imidodiphosphate; D011813:Quinuclidinyl Benzilate; D002217:Carbachol; D005033:Ethylmaleimide